1. Field of the Invention
This invention relates in general to spillways of hydraulic structures and more specifically to concrete step overlay protection for embankment dams.
2. Prior Art
Known in the art are dam spillways made in the form of open or closed channels communicating the reservoirs upstream and downstream of the dam and provided with water flow kinetic energy dissipators. Many attempts have been made to protect civil structures constructed of earth materials from erosion, whether the structures be a canal, waterway, or dam. Protection systems used on dams include grass linings, riprap, geotextiles and underlying grids, gabions, concrete block revetment systems, soil cement, and thick roller compacted concrete. These systems have been tested and used at various sites, but each has disadvantages or limitations.
Grass linings must have well established, uniform vegetative cover, which limits use in some climates. Unit discharges on 2.5:1 slopes are limited to less than 6.6 ft.sup.3 /s/ft, and on 20:1 slopes to 20 ft.sup.3 /s/ft. Small irregularity in the vegetative cover greatly increases the erosion or failure potential.
Scale model testing has been done on riprap to determine the stability of riprap on slopes up to 5:1. This modeling has shown that riprap scaled to represent 6-to 24-inch diameter rock, was suspended and washed downstream under the scaled unit discharge of 40 ft.sup.3 /s/ft. No analytical method is available to accurately predict the behavior of riprap protection with enough confidence to recommend its use as protection from overtopping flows of any significant magnitude.
Geotextiles, both with and without cover, and grids filled with gravel, placed on slopes from 2:1 to 4:1, were tested to failure, in large facilities, at unit discharges of 25 ft.sup.3 /s/ft and velocities about 22 ft/s. Failure occurred due to poor anchorage or stretching of the material.
Gabions are wire baskets filled with rock and anchored to slopes for erosion protection. They may perform well if anchored properly, but do undergo considerable deformation under flow conditions. Gabions should only be used up to tested velocities of 24 ft/s.
Concrete block revetment systems are generally cable-tied together, with grass cover over the voids, and anchored to the embankment. Two systems have been tested and are in use for overtopping protection, but may not be considered for velocities exceeding the test velocity of 26 ft/s. Simple concrete construction blocks filled with gravel have been used successfully up to velocities of 22 ft/s.
A wedge shaped concrete block was developed by Professor Yuri Pravdivets of the Moscow Institute of Civil Engineering in Russia. This block has been tested extensively, but is designed based upon block thickness vs. unit discharge. This leads to overdesign of the block based upon the test results of the instant invention.
Soil cement and roller compacted concrete (RCC) have proven to be very effective in protecting against erosion, however, their protection comes from the thickness of the concrete overlay alone. Applications are widespread but rely on the strength of the material and the cover thickness to provide protection. Subjecting the materials to high velocity flows would likely degrade the protective system. These techniques are economical only with placement of large quantities of material and require easy site access and may significantly impact the surrounding environment. The Russian block concept does not include interlocking pins which prevent buckling failures noted in European tests of the Russian design under some flow conditions.
Several prior art systems concerned with spillway design are available. U.S. Pat. No. 1,561,796 to Rehbock discloses a low, roof-shaped sill formed integrally with an apron. The sill is on the upstream side of its upper face and is provided with a series of teeth with a vertical upstream face and a gently sloping downstream face. The rapidly flowing part of the stream in the vicinity of the bed is gently deviated upwards by means of the toothed sill. The gently ascending streams of water flowing through the gaps between the teeth, prevent the main current from descending too rapidly to the bed and from affecting the ground.
U.S. Pat. No. 2,171,560 to Holmes discloses a method for fishway collection systems. U.S. Pat. No. 3,854,291 Perkins discloses a self cleaning filter for hydrological regeneration. The invention provides for a plurality of holding dams mounted in a stream and in which each holding dam is formed with a filter portion which receives the principle polluted liquid carried by water tight sewage conduits. The downstream side of the wall is provided with aeration troughs for adding air to the liquid as it flows past the dam.
U.S. Pat. No. 4,352,593 to Iskra et al discloses a dam spillway to pass water over the crest from a forebay into an afterbay and comprises a mixing chamber communicating with a diffuser, said mixing chamber has an intake arrangement ensuring formation downstream of the diffuser of a flotation zone with a froth collector installed at the end, the intake arrangement includes a water flow divider, a water breaking grid and air intake ducts, the divider being installed above the chamber and made in the form of a screen composed of chutes, the water grid of the intake arrangement composed of bluff members is provided in the inlet portion of the chamber, the air intake ducts of the intake arrangement are made in a wall of the mixing chamber below the grid in close proximity thereof.
There are several manufacturers of other types of concrete block revetment systems (Armorflex, Petraflex, Tri-Lock, etc.) that have limited applicability for dam overtopping protection. Most of these systems were designed to prevent river bank erosion.
In summary, the applicability of other known embankment protection systems are limited to providing erosion protection against low velocity flows, or flatter slope applications, or utilize mass concrete placement. The other known art most similar to the instant invention is the Russian wedge-shaped block design which has a fixed shape that does not serve to optimize block stability or energy dissipation of the flow.